This invention relates, in general, to digital electronic memories and, more specifically, to digital radio frequency memories suitable for use in radar countermeasures equipment.
Active radar jammers are used in the field of electronic countermeasures to confuse or counter a system originating radar signals. In some situations, it is desirable to return signals to the radar system which are exact copies of the arriving radar signal. In other situations, it is desirable to return signals to the radar system which have characteristics other than that of the received radar signal in order to further confuse the radar system. In any event, it is usually necessary for the countermeasure system to store the received radar signal and reproduce it at a later time.
Previously, delay lines of various types have been used to effectively store the received radar signal for a short period of time and make the stored radar signal available at the later time. One of the disadvantages of delay lines is that the delay cannot be electronically changed easily and that it is difficult to obtain reasonably long delay periods. An improvement over the delay line technology has been achieved by the use of digital radio frequency memories (DRFMs) which convert RF signals down to a lower IF frequency for storage into a digital memory device. The digital memory can be controlled similarly to the digital memory of a computer and the stored values representing the radar signal can be recalled at any time delay desired. Further manipulation of the digital values to produce changes in the replicated signal are also conveniently done by digital processes.
The usual method of employing DRFMs for storing radio frequency signals uses a two-channel (quadrature) memory system, usually denoted as the I and Q channels, with each channel storing digital values representing the RF signal displaced 90 degrees in phase. The maximum usable instantaneous bandwidth (IBW) of the memory system is one-half the sampling rate of the digital converters used in the DRFM, based upon the limitations governed by the Nyquist Sampling Theorem. To improve the amplitude dynamic range of the DRFM, to maximize the instantaneous bandwidth, and to minimize the digital storage bit requirements needed, one-bit sampling techniques are used in most DRFMs. Although one-bit sampling provides the advantages of increased dynamic range and reduced storage requirements, a large number of spurious frequencies, or unwanted spectral lines (spurs), are produced in the IF signal and ultimately in the reproduced RF signal because of the mixing processes used in the DRFM.
Having a large instantaneous bandwidth is advantageous from the standpoint that it allows radar signals over a wider range to be detected, stored and jammed by the countermeasure equipment. According to the prior art, the most efficient way to obtain reasonably large bandwidths was to use the I-Q two-channel storage system which stores the received signal into two different memory banks. However, the cost and size of such a two-channel system is a disadvantage in that a large amount of hardware is necessary to implement the system. Also, the two-channel system contains a non-performance region when the radar RF signal is close to the local oscillator frequency of the memory system.
Therefore, it is desirable, and it is an object of this invention, to provide an RF memory system which uses less digital memory than that of the conventional two-channel system, and which uses a smaller number of the associated components, such as couplers, mixers, converters, and splitters. It is also desirable to provide a digital memory system for RF signals which eliminates the non-performance region when the RF signal is near the local oscillator signal, and which eliminates or greatly reduces the number of spurs contained in the IF spectrum stored in the digital memory, while maintaining the advantages of one-bit sampling.